How Bronze is Made

The classic "lost wax" method of casting is used for bronze sculpture, sterling silver and gold jewelry. Here's how it's done:

1. The original sculpture (or jewelry design) is created , usually in plastilene, oil clay, polymer clay or various kinds of wax. Originals can also be wood, stone, marble, whatever, but I don't usually work in those materials.

2. Once the piece is completed, for some sculptures, the piece is readied for mold-making by having wax rods called "sprues" attached. These rods allow the hot air to escape when the bronze is poured, so there will be no air bubbles on the surface of the piece (and so the nose, ears, etc., will all actually be there -- trapped air can make an entire face disappear in a bubble!) The process of adding sprues is called "gating up. "

3. A rubber mold is made, either by me or by the foundry.

4. The rubber mold has a exterior plaster or fiberglass mold created (a "mother mold" ) to help support the rubber when wax is poured into it. The original sculpture is removed (and probably discarded unless it's stone or wood -- clay originals can get quite damaged in the mold-making process.)

5. A mold release agent (like a silicon spray) is applied to the interior of the mold before it's assembled, then the mold is put together, ready for pouring.

6. A special wax is melted and poured into the mold. The mold is turned to make sure all the surfaces are coated, then the wax is poured out. The process is repeated several more times until the correct thickness of wax is reached. (Small bronzes may be solid bronze, as is "Lazy Afternoon." Larger bronzes are usually hollow, with the bronze about 3/16" to 1/4" thick, depending on the size of the piece.) The molds for monumental-sized bronzes are made in small, manageable pieces, and the bronze is welded back together after casting.

7. The wax is removed from the mold and cleaned up, removing mold marks and making the piece as perfect as possible. (Bronze picks up every detail, including the artist's fingerprints!) This process is called "chasing" the wax. Sometimes the wax casting has to be cut in pieces for the bronze to pour better. The finished bronze will be welded back together and finished so nicely that you'll never know it started out in pieces.

8. The sprued wax casting is dipped in a liquid ceramic material called slurry. The slurry changes color (from orange to lime green, for instance) to indicate when it's dry. Once the wax has been dipped in the slurry, it's coated with ultra-fine sand. This process is repeated several times, with at least two coats of the finest sand as the first layers, to capture all the surface detail. Coarser sand is used for the final layers. The coated piece (which is said to be "invested" -- the slurry and sand are the "investment") is hung to dry.

9. Once thoroughly dry, the piece is put in a kiln (oven) that heats to a very high temperature, melting out the wax (thus the term "lost wax process"). The ceramic shell ("investment") is now a cleaned-out mold for bronze.

10. Red-hot to white-hot bronze is poured into the heated ceramic mold (still warm from the kiln, or heated so it won't crack when the molten metal is poured into it). Hotter bronze is used for pieces with lots of thin, fine detail (such as a tree with branches, for instance, or a stag with antlers). Pouring temperature ranges from 2000 to about 2600 degrees Fahrenheit.

11. When the bronze is cool, the ceramic shell (investment) is broken away, revealing the bronze. The piece is sandblasted to remove every speck of investment. The sprues and pour spout are cut off and the nubs ground down, and any mold marks or other blemishes polished away ("chasing" the bronze). If the sculpture was cut into pieces to be cast, it is welded together and weld marks ground off. The welds create bumps and a dark rainbow hue on the bronze. These nubs are also ground off. The color comes off in the final sand-blasting. At this point, the bronze is a bright brassy color.

12. The horse's hooves a(or the grass under his feet if I've included grass in the sculpture) are drilled and tapped for bolts or screws to attach him to his base.

13. Patina (acids which color the bronze and help define it) is applied. Usually a coat of liver sulfate goes on first and is allowed to darken a bit before being washed off. This makes the recesses of the piece shadowed. The main patina is applied and built up until it's the desired color. The high spots of the sculpture are often rubbed with Scotchbrite pads to keep the high spots light. I often have the highest edges buffed all the way back to bronze to give a glitter of gold to the piece - like sunlight on the edges of the horse's mane and tail. Molten clear paste wax is then applied, the piece is mounted on its base, and you have a finished bronze.

People often wonder why bronze is so expensive. Each step of the above process requires an experienced artisan. Mold-making is an art in itself. So are wax pouring, wax chasing, bronze chasing and patina work. That's five different specialties right there. The wax is burned out of the investment, so the investment has to hold up to the bronze pour or the week or more of preparation spent in creating that investment is lost. Investments do blow out from time to time, with molten bronze running out everywhere, which is why foundry workers wear Kevlar suits to protect them. The artist may take anywhere from a month to 3 to 5 months or more to create an original work of table-sized art. Monumental work can take up to a year or more to go from idea to finished product. If the mold-maker, wax chaser, bronze chaser or any of the others who work in this process don't do their jobs well, the entire piece may be ruined, and they'll have to start over.

When you see a bronze that's beautiful in every way, it's the product of not only the artist, but of many talented artisans at the foundry, the skill of the basemaker and the marble supplier.

Cold-Cast Bronze, Cold-Cast Porcelain, Resin and Papercasting

These sculptures are usually foundry-cast, produced in a pressure tank, as are their molds. The molds are silicon rubber with plaster mother molds like those used for bronze, but they're sometimes configured differently since these products are cast solid, not hollow. The pressure tank is used to get bubbles out of the mold (to ensure a smooth surface). The pressure tank is also used when casting the sculptures to get bubbles out of the finished sculpture and to be certain the casting material gets into every part of the mold. Both are resin products. Cold cast bronze includes bronze powder in the resin mix, while cold cast porcelain includes marble dust. The patina for the cold-cast bronze is built into the material used for casting, but there is a lot of hand-buffing, and additional patina work done to get exactly the right look. Cold-cast porcelain is sandblasted to get the soft look of bisque porcelain. The finished pieces have holes drilled in them for mounting. The finished pieces of both products are then often sealed with clear polyurethane to protect the finish.

I plan to do some sculptures in fine art acrylic (this material looks like ice sculpture, very elegant). The process for producing this water-clear material is similar to that for the cold-cast bronze or porcelain above, but with different material, and the acrylic is cast in an autoclave (vacuum with heat) rather than just a vacuum tank. Getting the acrylic to pour with absolutely no bubbles is the main concern. The piece is then sandblasted (where you want it to look frosted) or polished (where you want it to be clear).

The resins used in my reliefs (wall art) are cast in my home studio instead of the foundry. I make the molds and cast those myself because reliefs are simpler to mold and not as much room is required, nor is there as much requirement for special equipment as there is for the kind of resin poured at the foundry.

Papercastings are made with archival quality cotton linter. The paper comes as pulp with certain chemicals included to facilitate casting. The mold has to be at least 1" deep (or I have to build sides) to hold the pulp and water mixture. The mixture is poured in, magic words are said and art happens! :-) It takes practice and patience to get it right, believe me -- I threw away quite a few "mistakes" while I was learning the process. It takes several days for the papercasting to dry thoroughly, depending on how humid the weather is. The mold being out of use for so long while each casting dries is one of the reasons the papercastings sell for higher prices than the resins.

I've explained these processes to satisfy your curiosity (hey, I'm a curious person myself!), but believe me, none of this is as simple as it sounds! The first difficulty is in producing a good sculpture (but that's also the fun part!). Mold making is a very difficult art to master, and I do make my own molds for my wall art. Pouring wax without turbulence showing is difficult, too. There's a lot more to making sculpture than playing in clay! But it sure is FUN!